Rollover stability control for an automotive vehicle using front wheel actuators

ABSTRACT

A stability control system ( 24 ) for an automotive vehicle as includes a plurality of sensors ( 28 - 39 ) sensing the dynamic conditions of the vehicle and a controller controls a steering force to reduce a tire moment so the net moment of the vehicle is counter to the roll direction. The sensors may include a speed sensor ( 30 ), a lateral acceleration sensor ( 32 ), a roll rate sensor ( 34 ), a yaw rate sensor ( 20 ) and a longitudinal acceleration sensor ( 36 ). The controller ( 26 ) is coupled to the speed sensor ( 30 ), the lateral acceleration sensor ( 32 ), the roll rate sensor ( 34 ), the yaw rate sensor ( 28 ) and a longitudinal acceleration sensor ( 36 ). The controller ( 26 ) determines a roll angle estimate in response to lateral acceleration, longitudinal acceleration, roll rate, vehicle speed, and yaw rate. The controller ( 26 ) changes a tire force vector using by changing the direction and/or force of the steering actuator in response to the relative roll angle estimate.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation in part of U.S. patentapplication Ser. No. 09/468,234 entitled “ROLL OVER STABILITY CONTROLFOR AN AUTOMOTIVE VEHICLE” filed on Dec. 21, 1999.

BACKGROUND OF INVENTION

[0002] 1. Technical Field

[0003] The present invention relates generally to a dynamic behaviorcontrol apparatus for an automotive vehicle, and more specifically, to amethod and apparatus for controlling the roll characteristics of thevehicle by controlling the steering direction of the vehicle.

[0004] 2. Background

[0005] Dynamic control systems for automotive vehicles have recentlybegun to be offered on various products. Dynamic control systemstypically control the yaw of the vehicle by controlling the brakingeffort at the various wheels of the vehicle. Yaw control systemstypically compare the desired direction of the vehicle based upon thesteering wheel angle and the direction of travel. By regulating theamount of braking at each corner of the vehicle, the desired directionof travel may be maintained. Typically, the dynamic control systems donot address roll of the vehicle. For high profile vehicles inparticular, it would be desirable to control the roll overcharacteristic of the vehicle to maintain the vehicle position withrespect to the road. That is, it is desirable to maintain contact ofeach of the four tires of the vehicle on the road.

[0006] Vehicle rollover and tilt control (or body roll) aredistinguishable dynamic characteristics. Tilt control maintains thevehicle body on a plane or nearly on a plane parallel to the roadsurface. Roll over control is maintaining the vehicle wheels on the roadsurface. One system of tilt control is described in U.S. Pat. No.5,869,943. The ″943 patent uses the combination of yaw control and tiltcontrol to maintain the vehicle body horizontal while turning. Thesystem is used in conjunction with the front outside wheels only. Tocontrol tilt, a brake force is applied to the front outside wheels of aturn. In certain maneuvers, however, application of brakes may not bedesirable. It would therefore be desirable to provide a roll stabilitysystem that detects a potential rollover condition and temporarilyapplies steering in a desired direction to counter rollover.

SUMMARY OF INVENTION

[0007] It is therefore an object of the invention to provide a rollcontrol system for use in a vehicle using steering direction.

[0008] In one aspect of the invention, stability control system for anautomotive vehicle includes a plurality of sensors sensing the dynamicconditions of the vehicle and a controller that controls direction toreduce a tire moment so the net moment of the vehicle is counter to theroll direction. The sensors may include a speed sensor, a lateralacceleration sensor, a longitudinal acceleration sensor, a roll ratesensor, and a yaw rate sensor. A controller is coupled to the sensors todetermine a roll angle estimate. The controller determines the directionand steering effort change in response to the relative roll angleestimate to counter roll.

[0009] In a further aspect of the invention, a method of controllingroll stability of the vehicle comprises the steps of:

[0010] determining a roll angle estimate in response to a rolloversensor; and controlling at least one of the front steering actuators todetermining a tire force vector in response to the relative roll angleestimate.

[0011] One advantage of the invention is that such systems may be easilyimplemented into a steer-by-wire system.

[0012] Other objects and features of the present invention will becomeapparent when viewed in light of the detailed description of thepreferred embodiment when taken in conjunction with the attacheddrawings and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 is a diagrammatic rear view of a vehicle with force vectorsnot having a roll stability system according to the present invention.

[0014]FIG. 2 is a diagrammatic rear view of a vehicle with force vectorshaving a roll stability system according to the present invention.

[0015]FIG. 3 is a block diagram of a roll stability system according tothe present invention.

[0016]FIG. 4 is a flow chart of a yaw rate determination according tothe present invention.

[0017]FIG. 5 is a flow chart of roll rate determination according to thepresent invention.

[0018]FIG. 6 is a flow chart of a lateral acceleration determinationaccording to the present invention.

[0019]FIG. 7 is a flow chart of chassis roll angle estimation andcompensation.

[0020]FIG. 8 is a flow chart of a relative roll calculation.

[0021]FIG. 9 is a flow chart of system feedback for the vehicleresulting in a change in steering effort.

[0022]FIG. 10 is a flow chart of system feedback for the left side ofthe vehicle.

[0023]FIG. 11 is a flow chart of another embodiment similar to that ofFIGS. 9 and 10 resulting in change in steering position.

DETAILED DESCRIPTION

[0024] Referring to FIG. 1, an automotive vehicle 10 without a rolloverstability system of the present invention is illustrated with thevarious forces and moments thereon during a rollover condition. Vehicle10 has right and left tires 12 and 13 respectively. The vehicle may alsohave a number of different types of steering configurations includinghaving each of the front and rear wheels configured with anindependently controllable actuator, the front and rear wheels having aconventional type system in which both of the front wheels arecontrolled together and both of the rear wheels are controlled together,a system having conventional front steering and independentlycontrollable rear steering for each of the wheels or vice versa.Variation of a control system for each will be described below.Generally, the vehicle has a weight represented as M*g at the center ofgravity of the vehicle. A gravity moment 14 acts about the center ofgravity (CG) in a counter-clockwise direction. A tire moment 16 acts ina clockwise direction about the center of gravity. Thus, the net moment18 acting upon the vehicle is in a clockwise direction and thusincreases the roll angle 20 of the vehicle. The lateral force 22 at thetire 12 on the ground (tire vector) is a significant force to the leftof the diagram capable of overturning the vehicle when left uncorrected.

[0025] Referring now to FIG. 2, a roll stability control system 24 isincluded within vehicle 10, which is in a roll condition. The forcesillustrated in FIG. 2 are given the same reference numerals as theforces and moments in FIG. 1. In FIG. 2, however, roll stabilitycontroller 24 reduces the tire moment 16 to provide a net moment 18 in acounter-clockwise direction. Thus, the tire vector or lateral force 22at tire 12 is reduced as well. This tendency allows the vehicle to tendtoward the horizontal and thus reduce angle 20.

[0026] Referring now to FIG. 3, roll stability control system 24 has acontroller 26 used for receiving information from a number of sensorswhich may include a yaw rate sensor 28, a speed sensor 30, a lateralacceleration sensor 32, a roll rate sensor 34, a steering angle sensor35, a longitudinal acceleration sensor 36, a pitch rate sensor 37 steer.Lateral acceleration, longitudinal acceleration, yaw rate, rollorientation and speed may be obtained using a global positioning system(GPS). Based upon inputs from the sensors, controller 26 controls a tireforce vector by steering control 38 as will be further described belowor changing the steering angle. Depending on the desired sensitivity ofthe system and various other factors, not all the sensors 28-37 may beused in a commercial embodiment.

[0027] Roll rate sensor 34 and pitch rate sensor 37 may sense the rollcondition of the vehicle based on sensing the height of one or morepoints on the vehicle relative to the road surface. Sensors that may beused to achieve this include a lidar or radar-based proximity sensor, alaser-based proximity sensor and a sonar-based proximity sensor.

[0028] Roll rate sensor 34 and pitch rate sensor 37 may also sense theroll condition based on sensing the linear or rotational relativedisplacement or displacement velocity of one or more of the suspensionchassis components which may include a linear height or travel sensor, arotary height or travel sensor, a wheel speed sensor, a steering wheelposition sensor, a steering wheel velocity sensor and a driver headingcommand input from an electronic component that may include steer bywire using a hand wheel or joy stick.

[0029] The roll condition may also be sensed by sensing the force ortorque associated with the loading condition of one or more suspensionor chassis components including a pressure transducer in an activesuspension, a shock absorber sensor such as a load cell, a strain gauge,the steering system absolute or relative motor load, the steering systempressure of the hydraulic lines, a tire lateral force sensor or sensors,a longitudinal tire force sensor, a vertical tire force sensor or a tiresidewall torsion sensor.

[0030] The roll condition of the vehicle may also be established by oneor more of the following translational or rotational positions,velocities or accelerations of the vehicle including a roll gyro, theroll rate sensor 34, the yaw rate sensor 28, the lateral accelerationsensor 32, a vertical acceleration sensor, a vehicle longitudinalacceleration sensor, lateral or vertical speed sensor including awheel-based speed sensor, a radar-based speed sensor, a sonar-basedspeed sensor, a laser-based speed sensor or an opticalbased speedsensor.

[0031] Steering control 38 may control the position of the front rightwheel actuator 40A, the front left wheel actuator 40B, the rear leftwheel actuator 40C, and the right rear wheel actuator 40D. Although asdescribed above, two or more of the actuators may be simultaneouslycontrolled. For example, in a rack-and-pinion system, the two wheelscoupled thereto are simultaneously controlled. Based on the inputs fromsensors 28 through 39, controller 26 determines a roll condition andcontrols the steering position of the wheels. Controller 26 may also usebrake control 42 coupled to front right brakes 44A, front left brakes44B, rear left brakes 44C, and right rear brakes 44D. By using brakes inaddition to steering control some control benefits may be achieved. Thatis, controller 26 may be used to apply a brake force distribution to thebrake actuators in a manner described in U.S. Pat. No. 6,263,261 whichis hereby incorporated by reference.

[0032] Speed sensor 30 may be one of a variety of speed sensors known tothose skilled in the art. For example, a suitable speed sensor mayinclude a sensor at every wheel that is averaged by controller 26.Preferably, the controller translates the wheel speeds into the speed ofthe vehicle. Yaw rate, steering angle, wheel speed and possibly a slipangle estimate at each wheel may be translated back to the speed of thevehicle at the center of gravity (V_CG). Various other algorithms areknown to those skilled in the art. Speed may also be obtained from atransmission sensor. For example, if speed is determined while speedingup or braking around a corner, the lowest or highest wheel speed may benot used because of its error. Also, a transmission sensor may be usedto determine vehicle speed.

[0033] Referring now to FIG. 4, The yaw rate sensor 28 generates a rawyaw rate signal (YR_Raw). A yaw rate compensated and filtered signal(YR_CompFlt) is determined. The velocity of the vehicle at center ofgravity (V_CG), the yaw rate offset (YR_Offset) and the raw yaw ratesignal from the yaw rate sensor (YR_Raw) are used in a yaw rate offsetinitialization block 45 to determine an initial yaw rate offset. Becausethis is an iterative process, the yaw rate offset from the previouscalculation is used by yaw rate offset initialization block 45. If thevehicle is not moving as during startup, the yaw rate offset signal isthat value which results in a compensated yaw rate of zero. This yawrate offset signal helps provide an accurate reading. For example, ifthe vehicle is at rest, the yaw rate signal should be zero. However, ifthe vehicle is reading a yaw rate value then that yaw rate value is usedas the yaw rate offset. The yaw rate offset signal along with the rawyaw rate signal is used in the anti-windup logic block 46. Theanti-windup logic block 46 is used to cancel drift in the yaw ratesignal. The yaw rate signal may have drift over time due to temperatureor other environmental factors. The anti-windup logic block also helpscompensate for when the vehicle is traveling constantly in a turn for arelatively long period. The anti-windup logic block 46 generates eithera positive compensation OK signal (Pos Comp OK) or a negativecompensation OK signal (Neg Comp OK). Positive and negative in thismanner have been arbitrarily chosen to be the right and left directionwith respect to the forward direction of the vehicle, respectively. Thepositive compensation OK signal, the negative compensation OK signal andthe yaw rate offset signal are inputs to yaw rate offset compensationlogic block 47.

[0034] The yaw rate offset compensation logic block 47 is used to takedata over a long period of time. The data over time should have anaverage yaw of zero. This calculation may be done over a number ofminutes. A yaw rate offset signal is generated by yaw rate offsetcompensation logic 47. A summing block 48 sums the raw yaw rate signaland the yaw rate offset signal to obtain a yaw rate compensated signal(YR_Comp).

[0035] A low pass filter 49 is used to filter the yaw rate compensatedsignal for noise. A suitable cutoff frequency for low pass filter 49 is20 Hz.

[0036] Referring now to FIG. 5, a roll rate compensated and filteredsignal (RR_CompFlt). The roll rate compensated and filtered signal isgenerated in a similar manner to that described above with respect toyaw rate. A roll rate offset initialization block 50 receives thevelocity at center of gravity signal and a roll rate offset signal. Theroll rate offset signal is generated from a previous iteration. Like theyaw rate, when the vehicle is at rest such as during startup, the rollrate offset is chosen so that the roll rate signal is zero.

[0037] A roll rate offset compensation logic block 52 receives theinitialized roll rate offset signal. The roll rate offset compensationlogic generates a roll rate offset signal which is combined with theroll rate raw signal obtained from the roll rate sensor in a summingblock 54. A roll rate compensated signal (RR_Comp) is generated. Theroll rate compensated signal is filtered in low pass filter 56 to obtainthe roll rate compensated and filtered signal that will be used in latercalculations.

[0038] Referring now to FIG. 6, the raw lateral acceleration signal (LatAcc Raw) is obtained from lateral acceleration sensor 32. The rawlateral acceleration signal is filtered by a low pass filter to obtainthe filtered lateral acceleration signal (Lat Acc Flt). The filter, forexample, may be a 20 Hz low pass filter.

[0039] Referring now to FIG. 7, a roll angle estimation signal(RollAngleEst) is determined by chassis roll estimation and compensationprocedure 62. Block 64 is used to obtain a longitudinal vehicle speedestimation at the center of gravity of the vehicle. Various signals areused to determine the longitudinal vehicle speed at the center ofgravity including the velocity of the vehicle at center of gravitydetermined in a previous loop, the compensated and filtered yaw ratesignal determined in FIG. 4, the steering angle, the body slip angle,the front left wheel speed, the front right wheel speed, the rear leftwheel speed, and the rear right wheel speed.

[0040] The new velocity of the center of gravity of the vehicle is aninput to body roll angle initialization block 66. Other inputs to bodyroll angle initialization block 66 include roll angle estimate from theprevious loop and a filtered lateral acceleration signal derived in FIG.6. An updated roll angle estimate is obtained from body roll angleinitialization. The updated roll angle estimate, the compensated andfiltered roll rate determination from FIG. 5, and the time of the loopis used in body roll angle integration block 68. The updated roll angleestimate is equal to the loop time multiplied by the compensated andfiltered roll rate which is added to the previous roll angle estimateobtained in block 66. The updated roll angle estimate is an input toroll angle estimate offset compensation block 70.

[0041] The velocity at the center of gravity of the vehicle is also aninput to instantaneous roll angle reference block 72. Other inputs toinstantaneous roll angle reference block 72 include the compensated andfiltered yaw rate from FIG. 4 and the filtered lateral accelerationsignal from FIG. 6. The following formula is used to determine areference roll angle:

Reference Roll Angle=ARCSin[1/g(VCG*YRCompFlt-LatAccFlt)]|

[0042] Where g is the gravitational constant 9.81 m/s²

[0043] The reference roll angle from block 72 is also an input to rollangle estimate offset compensation. The updated roll angle estimation isgiven by the formula:${RollAngleEst} = {{{RollAngleEst}\left( {{from}\quad {Blcok}\quad 68} \right)} + {ReferenceRollAngle} - {{RollAngleEst}\quad {Block}\quad 68\quad \frac{looptime}{Tau}}}$

[0044] Where Tau is a time constant and may be a function of steeringvelocity, LatAcc and V-CG. A suitable time constant may, for example, bebetween 1 and 30 seconds depending on the vehicle condition.

[0045] Referring now to FIG. 8, a relative roll angle estimation(RelativeRollAngleEst) and a road bank angle estimate signal isdetermined. The first step of the relative roll angle calculationinvolves the determination of road bank angle compensation time constant(Tau) block 72. The velocity at the center of gravity, the steeringvelocity and the filtered lateral acceleration signal from FIG. 6 areused as inputs. A compensated and filtered roll rate (RR_CompFlt) isused as an input to a differentiator 74 to determine the rollacceleration (Roll Acc). Differentiator 74 takes the difference betweenthe compensated and filtered roll rate signal from the previous loop andthe compensated and filtered roll rate from the current loop divided bythe loop time to attain the roll acceleration. The roll accelerationsignal is coupled to a low pass filter 76. The filtered rollacceleration signal (Roll Acc Flt), roll angle estimate, the filteredlateral acceleration signal and the loop time are coupled to chassisrelative roll observer block 78. The chassis roll observer 78 determinesthe model roll angle estimation (Model Roll Angle Est). The model rollangle is a stable estimation of the roll dynamics of the vehicle whichallows the estimates to converge to a stable condition over time.

[0046] From the model roll angle estimation from block 78, the initialrelative roll angle estimation from block 72, a road bank angleinitialization from a block 79 loop time and a roll angle estimate, roadbank angle compensation block 80 determines a new road bank angleestimate. The formula for road bank angle is:${RoadBankAngleEst} = {\frac{LoopTime}{TauRoad\_ Bank}*\left( {{RollAngleEst} - \begin{pmatrix}{{ModelRollAngle} +} \\{RoadBankAngleEst}\end{pmatrix}} \right)}$

[0047] The roll angle estimate may be summed with the road bank angleestimate from block 80 in summer 82 to obtain a relative roll angleestimate. The road bank angle estimate may be used by other dynamiccontrol systems.

[0048] Referring now to FIG. 9, the relative roll angle estimate fromFIG. 8 and a relative roll deadband are summed in summer 84 to obtain anupper roll error. The upper roll error is amplified in KP_Roll Amplifier86 and is coupled to summer 88. The roll rate compensated and filteredsignal from FIG. 5 is coupled to KD_Roll Amplifier 90. The amplifiedroll rate signal is coupled to summer 88. The filtered roll accelerationsignal from block 8 is coupled to KDD_Roll Amplifier 82. The amplifiedsignal is also coupled to summer 88.

[0049] The proportioned sum of the amplified signals is the right sidesteering tire correction. The steering actuator control signals arecalculated from the tire corrections, the front steer angle(s) (or thesteering actuator positions), the vehicle side slip angle, the vehicleyaw rate and vehicle speed. Increased accuracy and robustness can beachieved by including tire normal load estimates and/or tire slipratios. In the steering angle and effort correction block 94, the tireslip angles are calculated and used to determine the corrections to thefront steer angles that will reduce the tire lateral forces and reducethe vehicle roll angle. Block 94 also calculates the actuator controlsignals necessary to achieve the desired tire steering corrections.

[0050] The measured steering actuator positions are inputs to block 94.The change in the actuator direction and effort amounts and duration areoutputs of block 94. The block 94 determines the appropriate directionand force amount to apply to the steering actuators to prevent roll.

[0051] The output of block 94 is used by the steering controller 38 ofFIG. 3 to apply the desired steering to the front and/or rear wheelsdepending on the type of steering system. The steering controllerfactors in inputs such as the current steering position and the dynamicsof the vehicle. Other inputs may include inputs from other dynamiccontrol systems such as a yaw control system. In a production readyembodiment, the vehicle design characteristics will be factored into thedesired control based on the sensor outputs.

[0052] The bottom portion of FIG. 9 is similar to the top, however, thesigns are changed to reflect that the left side of the vehicle is anegative side of the vehicle. Therefore, relative roll angle estimateand relative roll deadband are purely summed together 96 in summingblock 96 to obtain the lower roll error. The lower roll error is passedthrough KP_Roll amplifier 98. The compensated and filtered roll rate ispassed through KDD_Roll amplifier 100 and the filtered roll accelerationsignal is passed through KDD_Roll amplifier 102. The inverse of thesignals from amplifiers 98, 100 and 102 are input and summed in summer104 to obtain the desired left actuator control.

[0053] By properly applying a desired steering control to the vehicle,the tire moment is reduced and the net moment of the vehicle is counterto a roll direction to reduce the roll angle and maintain the vehicle ina horizontal plane.

[0054] In operation, various types of steering control may be performeddepending on the vehicle characteristics and the steering system. Forexample, as described above a rack system may be controlled to provide adesired change in the rear steering angle temporarily to preventrollover while leaving the front wheels unchanged. Of course, thedirection of the front wheels could also be change when the reardirection is changed.

[0055] In a system having independently actuable front wheels, therelative steering angle between the front wheels may be changed inresponse to detected roll by steering control 38 without changing theposition or controlling the position of the rear wheel. This may be doneby independent control of the front wheels or simultaneous control ofthe front wheels.

[0056] In a system having independently actuable rear wheels, therelative steering angle between the front wheels may be changed inresponse to detected roll by steering control 38 without changing theposition or controlling the position of the front wheels. This may bedone by independent control of the rear wheels or simultaneous controlof the rear wheels.

[0057] As described above the longitudinal acceleration sensor and apitch rate sensor may be incorporated into the above tire force vectordetermination. These sensors may be used as a verification as well as anintegral part of the calculations. For example, the pitch rate or thelongitudinal acceleration or both can be used to construct a vehiclepitch angle estimate.

[0058] This estimate along with its derivative can be used to improvethe calculation of the vehicle roll angle. An example of how the rate ofchange of the vehicle roll angle using theses variables may beconstructed is:

GlobalRR≈RRComp_Flt+PitchRateCompFlt (−YawRate+Sin(GlobalRollAngleEst)*Tan(VehiclePitchAngleEst))+(YawRateCompFlt*Cos(GlobalRR)*Tan(PitchAngleEst))

[0059] Where PitchRateCompFlt is a compensated and filtered pitch ratesignal, GlobalRollAngleEst is an estimated global roll angle,VehiclePitchAngleEst is an estimated vehicle pitch angle estimate, andGlobalRR is a global roll rate signal. Of course, those skilled in theart may vary the above based upon various other factors depending on theparticular system needs.

[0060] While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. A stability control system for an automotive vehicle having a frontsteering system comprising: a front right wheel actuator; and a frontleft wheel actuator; a front right wheel position sensor generating afront right position signal and a front left wheel position sensorgenerating a front left wheel position signal; a rollover sensor forproducing a rollover signal in response to an impending rollover of thevehicle; and a controller coupled to said front right wheel positionsensor, said front left wheel position sensor, said rollover sensor,said front right wheel actuator and said front left wheel actuator, saidcontroller generating a front right wheel actuator signal and a frontleft wheel actuator signal in response to said rollover signal, saidfront right wheel actuator signal and said front left wheel actuatorsignal controlling said respective front left and front right steeringactuator to prevent the vehicle from rolling over. 2.A stability controlsystem as recited in claim 1 wherein said rollover sensor comprises oneor more selected from the group of a lateral acceleration sensor, a rollrate sensor, and a load sensor. 3.A stability control system as recitedin claim 2 further comprising a sensor selected from the group of asteering angle sensor, a speed sensor, a yaw rate sensor, a longitudinalacceleration sensor and a pitch rate sensor. 4.A stability controlsystem as recited in claim 1 wherein said controller changes a tireforce vector by changing a direction and front steering force of saidfront right actuator. 5.A stability control system as recited in claim 1wherein said controller changes a tire force vector by changing adirection and front steering force of said front left actuator. 6.Astability control system as recited in claim 1 wherein said controllerchanges a tire force vector by changing a relative direction betweensaid right front actuator and said left front actuator. 7.A stabilitycontrol system as recited in claim 1 further comprising a plurality ofbrake actuators, said controller controlling said brake actuators inresponse to said rollover signal. 8.A stability control system asrecited in claim 1 further comprising a rear wheel steering actuator,said controller controlling said rear wheel steering actuator inresponse to said rollover signal. 9.A method of controlling rollstability of a vehicle front steering actuator comprising: sensing theposition of the front steering actuator; sensing an impending rollover;generating a tire moment counter to a roll direction by controlling thefront actuator in response to said impending rollover and said positionof the front steering actuator.
 10. A method as recited in claim 9wherein said vehicle comprises a right front actuator and a left frontactuator and said step of generating comprises generating a tire momentby changing a relative direction between said right front actuator andsaid left front actuator.
 11. A method as recited in claim 9 whereinsaid step of sensing an impending rollover comprises at least one stepselected from the group of: determining a roll rate for the vehicle;determining a lateral acceleration for the vehicle; and determining arelative load at one corner of the vehicle.
 12. A method of controllingroll stability of a vehicle having a front steering system havingindependently actuable front steering actuator comprising the steps of:determining a roll angle estimate in response to a rollover sensor; andcontrolling at least one of the front steering actuators to determininga tire force vector in response to the relative roll angle estimate.13.A method as recited in claim 12 wherein said vehicle comprises aright front actuator and a left front actuator and said step ofgenerating comprises generating a tire moment by changing a relativedirection between said right front actuator and said left frontactuator.
 14. A method as recited in claim 12 wherein said step ofdetermining a roll angle estimate comprises the steps of: determining aroll rate for the vehicle; determining a lateral acceleration for thevehicle; and determining a relative load at one corner of the vehicle.15.A method as recited in claim 10 further comprising: sensing theposition of rear wheel steering actuator; and generating the tire momentcounter to the roll direction by controlling the rear wheel steeringactuator in response to said impending rollover and said position of therear wheel steering actuator.